Method and apparatus for multiplex addressing of a ferro-electric liquid crystal display

ABSTRACT

A ferro-electric liquid crystal display is multiplex addressed by strobe waveform applied in sequence to each electrode in one set of electrodes coincidently with data waveforms applied to a second set of electrodes. Liquid crystal material in the display is switched by a d.c. pulse of appropriate polarity, amplitude and time. The strobe waveforms have first and second pulse pairs, each pulse pair comprising two pulses of different amplitude and the same or different sign. The pulse pairs are similar but of opposite sign. Data waveforms are rectangular waveforms of opposite sign. The amplitude and ratio of leading pulse to trailing pulse in each strobe pulse pair are adjusted to obtain the desired switching and contrast. Compensation for temperature changes is arranged by measuring the temperature of the liquid crystal material and using the value obtained to adjust the amplitude value of the leading pulse in each strobe pulse pair.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the multiplex addressing of ferro-electricliquid crystal displays. Such displays may use a chiral smectic C, I,and F liquid crystal material.

2. Discussion of Prior Art

Liquid crystal display devices commonly comprise a thin layer of aliquid crystal material contained between two glass slides. Electrodestructures on the inner faces of these slides enable an electric fieldto be applied across the liquid crystal layer thereby changing itsmolecular alignment. Many different types of displays have been madeusing nematic and cholesteric liquid crystal material. Both these typesof material are operated between a field ON state and a field OFF state;i.e. displays are operated by switching a field on and off.

A more recent type of display uses a ferroelectric chiral smectic C, I,and F liquid crystal material in which liquid crystal molecules adoptone of two possible field 0N states depending on the polarity of appliedfield. These displays are thus switched between the two states by pulsesof appropriate polarity. In a zero applied field the molecules adopt anintermediate, configuration. Chiral smectic displays offer very fastswitching with an amount of bistability. Examples of chiral smecticdisplays are described in G.B. No. 2,163,273, G.B. No. 2,159,635, (U.S.Pat. No. 4,712,873) G.B. No. 2,166,256, (U.S. Pat. No. 4,722,594) G.B.No. 2,157,451, (U.S. Pat. No. 4,720,193), U.S. Pat. No. 4,536,059, U.S.Pat. No. 4,367,924, G.B. P.A. No. 86/08,114-P.C.T. No. G.B. 87/00,222,(G.B. 2,209,610 corresponds to U.S. Ser. No. 07/279,553) G.B. P.A. No.08,115-P.C.T. No.87/00,221, G.B. (G.B. 2,210,468 corresponds to U.S.Pat. No. 4,969,719) P.A. No. 08,116-P.C.T. 87/00,220 (G.B. 2,210,469corresponds to U.S. Pat. No. 4,997,264).

There are a number of known systems for multiplex addressing chiralsmectic displays; see for example article by Harada et al 1985 S.I.D.Paper 8.4 pp 131-134, and Lagerwall et al 1985 I.D.R.C. pp 213-221. Inthis system a switching pulse is immediately preceeded by an equal andopposite polarity pulse which switches to the opposite state. Thepurpose of an opposite pulse followed by the wanted switching pulse isto ensure net d.c. at the liquid crystal material. See also GB2,173,336A (U.S. Pat No. 4,705,345) and GB 2,173,629A.

A disadvantage of this system is a reduced switching time. Also thematerial sometimes fails to switch to the wanted state but stays in anopposite switched state. This gives inverted contrast which undercertain conditions could be difficult to control in a complex display.

SUMMARY OF THE INVENTION

According to this invention a method of multiplex addressing s ferroelectric liquid crystal matrix display formed by the intersections of afirst set of electrodes and a second set of electrodes comprises thesteps of:

applying a strobe waveform to each electrode in sequence in the firstset of electrodes, said strobe waveform comprising a first pair ofstrobe pulses of different amplitude followed by a second pair of pulsesof similar amplitude but different sign to the first pair of strobepulses,

applying one of two data waveforms to each electrode in the second setof electrodes coincidently with strobe waveform, both data waveformsbeing rectangular waveforms of alternate positive and negative valueswith one data waveform the inverse of the other data waveform,

whereby each intersection is addressed with a d.c. pulse of appropriatesign and magnitude to turn that intersection to a desired display stateonce per complete display address period and an overall net zero d.c.value in each complete display address period.

According to this invention a multiplex addressed liquid crystal displaycomprises:

a liquid crystal cell including a layer of ferro-electric smectic liquidcrystal material contained between two walls each bearing a set ofelectrodes arranged to form collectively a matrix of addressableintersections,

driver circuits for applying data waveforms to one set of electrodes andstrobe waveforms to the other set of electrodes in a multiplexed manner,

waveform generators for generating data and strobe waveforms forapplying to the driver circuits,

means for controlling the order of data waveforms so that a desireddisplay pattern is obtained,

Characterised by:

a data waveform generator that generates two sets of waveforms of equalamplitude and frequency but opposite sign, each data waveform comprisingd.c. pulses of alternate sign,

a strobe waveform generator that generates strobe waveforms comprising afirst pair of strobe pulses of different amplitude followed by a secondpair of pulses of similar amplitude but different sign to the first pairof strobe pulses.

The strobe waveform may comprise two pairs of strobe pulses separated bya number of time periods when a zero strobe pulse is generated.Alternatively the second pair of strobe pulses may immediately followthe first pair.

Each pair of strobe pulses may be a pulse of one sign followed by spulse of the opposite sign. Alternatively in each pair both strobepulses may be of the same sign.

The amplitude of one strobe pulse in each pair is greater than, in anyproportion, the amplitude of the other strobe pulse.

The amplitude of the smaller strobe pulse in each pair may be the sameas or different from the amplitude of the data pulses.

BRIEF DESCRIPTION OF THE DRAWINGS

The amplitude and sign of the leading pulse in each strobe pulse pairmay be varied to provide satisfactory display operation over a widerange of temperatures.

The invention will now be described by way of example only withreference to the accompanying drawings of which:

FIG. 1 is a diagrammatic view of a time multiplex addressed x, y matrix;

FIG. 2 is s cross section of part of the display of FIG. 1 to anenlarged scale;

FIG. 3 is a view of an x, y matrix showing one pattern of ON elements;

FIG. 4 is waveform diagrams;

FIG. 5 is a graph showing a boundary between switching and non-switchingvalues of time and applied voltage amplitude.

FIG. 6 is a graph of applied voltage vs switching times for differentvalues of applied a.c. bias voltage;

FIG. 7 is a graph of applied voltage vs switching times for differentvalues of leading pulse ratio;

FIGS. 8(a)-8(b) shows waveform traces having positive and negativeleading pulse ratios as used for measurement of the curves shown in FIG.7;

FIG. 9 is a graph of applied voltage vs switching times for differentliquid crystal temperatures;

FIGS. 10, 11, 12 shows graphs of applied voltage vs switching times atdifferent temperatures and show the effect of varying leading pulseratios to provide temperature compensation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The display 1 shown in FIGS. 1, 2 comprises two glass walls 2, 3 spacedabout 1-6 μm apart by a spacer ring 4 and/or distributed spacers.

Electrode structures 5, 6 of transparent tin oxide are formed on theinner face of both walls. These electrodes are shown as row and columnforming an X, Y matrix but may be of other forms. For example, radialand curved shape for an r, θ display, or of segments form for a digitalseven bar display.

A layer 7 of liquid crystal material is contained between the walls 2, 3and spacer ring 4.

Polarisers 8, 9 are arranged in front of and behind the cell 1. Row 10and column 11 drivers apply voltage signals to the cell. Two sets ofwaveforms are generated for supplying the row and column drivers 10, 11.A strobe wave form generator 12 supplies row waveforms, and a datawaveform generator 13 supplies ON and OFF waveforms to the columndrivers 11. Overall control of timing and display format is controlledby a contrast logic unit 14. Temperature of the liquid crystal, layer 7,is measured by a thermocouple 15 whose output is fed to the strobegenerator 12. The thermocouple 15 output may be direct to the generatoror via a proportioning element 16 e.g. a programmed ROM chip to vary onepart of the strobe pulse waveform.

Prior to assembly the walls 2, 3 are surface treated by spinning on athin layer of polyamide or polyimide, drying and where appropriatecuring; then buffing with a soft cloth (e.g. rayon) in a singledirection R₁, R₂. This known treatment provides a surface alignment forliquid crystal molecules. The rubbing directions R₁, R₂ areantiparallel. Then suitable unidirectional voltages are applied themolecules director align along one of two directors D₁, D₂ depending onpolarity of the voltage. Typically the angle between D₁, D₂ is about45°. In the absence of an applied electric field the molecules adopt anintermediate alignment directions R₁, R₂ and the directions D₁, D₂.

The device may operate in a transmissive or reflective mode. In theformer light passing through the device e.g. from a tungsten bulb isselectively transmitted or blocked to form the desired display. In thereflective mode a mirror is placed behind the second polariser 9 toreflect ambient light back through the cell 1 and two polarisers. Bymaking the mirror partly reflecting the device may be operated both in atransmissive and reflective mode.

Pleochroic dyes may be added to the material 7. In this case only onepolariser is needed and the layer thickness may be 4-10 μm.

Suitable liquid crystal materials are:

catalogue references BDH-SCE 3 available from BDH, Poole, Dorset, and

19.6% CM8 (49% CC1+51% CC4)+80.4% H₁ ##STR1## Another mixture is LPM68=H1 (49.5%), AS 100 (49.5%), IGS 97(1%) H1=MB 8.5F+MB 80.5F+MB 70.7F(1:1:1)

AS100=PYR 709+PYR 9.09 (1:2) ##STR2## For a typical thickness of 2 μmthis material at 22° C. is switched by a d.c. pulse of + or - 50 voltsfor 100 μs. The two switched states D₁, D₂ may be arbitrarily defined as0N after receiving a positive pulse and OFF after receiving a negativepulse of sufficient magnitude. Polarisers 8, 9 are arranged with theirpolarisation axes perpendicular to one another and with one of the axesparallel to the director in one of the switched states.

In operation strobe waveforms are applied to each row in turn whilstappropriate 0N or OFF data waveform are applied to each columnelectrode. This provides a desired display pattern formed by some x, yintersection in an 0N state and other in an OFF state. Such addressingis termed multiplex addressing. The present invention is distinguishedfrom prior art systems by the shape of the applied waveforms.

FIG. 3 shows a 4 by 4 x, y matrix with 0N intersections indicated by asolid circle, elsewhere the display is OFF.

FIG. 4 shows the shape of data ON and OFF plus the shape of strobewaveforms. Each data and strobe pulse lasts for a period of one timeslot. As seen the strobe waveform is formed by two sets of pulse pairsseparated by a number of time slots where zero voltage is applied. Thesepairs are of opposite polarity. A+1 pulse is immediately followed by oneof -3; zero volts, i.e. earthed, is then applied until the end of afirst field period when a -1 volt pulse is followed by a +3 pulse. Astring of zero pulses complete a second field. A display is addressed byboth fields to provide the desired information. The length of bothfields and hence the number of time slots between pairs of pulses isdependent on the number of rows to be addressed. A larger number of rowsrequires a large number of time slots between the pairs of pulses.

Waveforms applied to each row and column, and to the resulting value ateach x, y intersection are shown in tabular form in Table 1. Row 1 isindicated by R1 etc; intersection of row 1 and column 1 is indicated byR1, C1 etc.

The values of applied voltage are adjusted such that +1 or -1 does notswitch the display. A +/- 3 or more value will switch the display.However the chiral smetic is sensitive to the amplitude time product asshown in FIG. 5. Therefore it is necessary to ensure that whensuccessive time slots are of the same polarity their amplitude timeproduct does not exceed the threshold for switching. The manner in whichboth voltage and time effect switching is shown in FIG. 5; values, abovethe curve give a switch effect. Note, the curve indicates whether or notswitching occurs from either ON or OFF state. The voltage values aremodulus voltages.

For the row 1 column 1 intersection a -2 amplitude followed by -1 isobtained in the first field time. Thus the actual value of -2 needs tobe kept as low as possible. At the beginning of field 2 a -2 isimmediately followed by +4 which is high enough to give a clear switchto an 0N state. Similarly, in row 1 column 2, a -4 value gives a clearswitch to an OFF state.

Strobe waveforms having values other than +/-1 and +/-3 may be chosen,for example Table 1(b) shows the effect obtained with strobe pulses of1, -2; -1, 2. Intersections receive maximum values of 3 proceeded by -2,or -3 preceeded by +2. The values -2, (or +2) start to turn theintersection to the OFF (or 0N) state whilst the 3 (or -3) fullyswitches the intersection to the desired 0N (or OFF) state.

Various other strobe waveforms and consequential intersection waveformsare shown in Tables 2 to 8.

Table 5-8 show how the two pairs of strobe pulses can be adjacent oneanother so that only one field is used per frame instead of the twofields of Tables 1 to 4. In all cases the relative values of each strobepulse and data pulse amplitude can be varied from that shown. Values of1 and 3 are merely by way of example only.

                                      TABLE 1 (a)                                 __________________________________________________________________________    Time Data                                                                     ON  1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1                     OFF -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1                      Strobe                                                                        R1  1  -3 0  0  0  0  0  0  -1 3  0  0  0  0  0  0  1  -3                     R2  0  0  1  -3 0  0  0  0  0  0  -1 3  0  0  0  0  0  0                      R3  0  0  0  0  1  -3 0  0  0  0  0  0  -1 3  0  0  0  0                      R4  0  0  0  0  0  0  1  -3 0  0  0  0  0  0  -1 3  0  0                      Waveform at column for the display of FIG. 3                                  C1  1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1                     C2  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1                      C3  -1 1  -1 1  1  -1 -1 1  -1 1  -1 1  1  -1 -1 1  -1 1                      C4  1  -1 1  -1 -1 1  1  -1 1  -1 1  -1 -1 1  1  -1 1  -1                     Waveform at x, y intersection for the display of FIG. 3                       R1C1                                                                              0  -2 -1 1  -1 1  -1 1  -2 4  -1 1  -1 1  -1 1  0  -2                     R2C2                                                                              1  -1 2  -4 1  -1 1  -1 1  -1 0  2  1  -1 1  -1 1  -1                     R3C3                                                                              1  -1 1  -1 0  -2 1  -1 1  -1 1  -1 -2 4  1  -1 1  1                      R3C4                                                                              -1 1  -1 1  2  -4 -1 1  -1 1  -1 1  0  2  -1 1  -1 1                      __________________________________________________________________________

                                      TABLE 1 (b)                                 __________________________________________________________________________    Data                                                                          ON  1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1                     OFF -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1                      Strobe                                                                        R1  1  -2 0  0  0  0  0  0  -1 2  0  0  0  0  0  0  1  -2                     R2  0  0  1  -2 0  0  0  0  0  0  -1 2  0  0  0  0  0  0                      R3  0  0  0  0  1  -2 0  0  0  0  0  0  -1 2  0  0  0  0                      R4  0  0  0  0  0  0  1  -2 0  0  0  0  0  0  -1 2  0  0                      Waveform at x, y intersection for the display of FIG. 3                       R1C1                                                                              0  -1 -1 1  -1 1  -1 1  -2 3  -1 1  -1 1  -1 1  0  -1                     R2C2                                                                              1  -1 2  -3 1  -1 1  -1 1  -1 0  1  1  -1 1  -1 1  -1                     R3C3                                                                              1  -1 1  -1 0  -1 1  -1 1  -1 1  -1 -2 3  1  -1 1  -1                     R3C4                                                                              -1 1  -1 1  2  -3 -1 1  -1 1  -1 1  0  1  -1 1  -1 1                      __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    Data                                                                          ON  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1                      OFF 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1                     Strobe                                                                        R1  -3 1  0  0  0  0  0  0  3  -1 0  0  0  0  0  0  -3 1                      R2  0  0  -3 1  0  0  0  0  0  0  3  -1 0  0  0  0  0  0                      R3  0  0  0  0  -3 1  0  0  0  0  0  0  3  -1 0  0  0  0                      R4  0  0  0  0  0  0  -3 1  0  0  0  0  0  0  3  -1 0  0                      Waveforms at columns for the display of FIG. 3                                 C1 -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1                      C2  1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1                     C3  1  -1 1  -1 -1 1  1  -1 1  -1 1  -1 -1 1  1  -1 1  -1                     C4  -1 1  -1 1  1  -1 -1 1  -1 1  -1 1  1  -1 -1 1  -1 1                      Waveform at x,y intersection for the display of FIG. 3                        R1C1                                                                              -2 0  1  -1 1  -1 1  -1 4  -2 1  -1 1  -1 1  -1 -2 0                      R2C2                                                                              -1 1  -4 2  -1 1  - 1                                                                              1  -1 1  2  0  -1 1  -1 1  -1 1                      R3C3                                                                              -1 1  -1 1  -2 0  -1 1  -1 1  -1 1  4  -2 -1 1  -1 1                      R3C4                                                                              1  -1 1  -1 -4 2  1  -1 1  -1 1  -1 2  0  1  -1 1  -1                     __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________    Data                                                                          ON 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1                OFF                                                                              -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1      -1                                                                     1                       Strobe                                                                        R1  -1 -3  0  0  0   0  0  0  1  3   0  0  0  0  0   0  0  0                  R2  0  0   -1 -3 0   0  0  0  0  0   1  3  0  0  0   0  0  0                  R3  0  0   0  0  -1  -3 0  0  0  0   0  0  1  3  0   0  0  0                  R4  0  0   0  0  0   0  -1 -3 0  0   0  0  0  0  1   3  0  0                  Waveforms at x,y intersections for the display of FIG. 3                      R1C1                                                                              -2 -2  -1 1  -1  1  -1 1  0  4   -1 1  -1 1  -1  1  -2 2                  R2C2                                                                              1  -1  0  -4 1   -1 1  -1 1  -1  2  2  1  -1 1   -1 1  -1                 R3C3                                                                              1  -1  1  -1 -2  -2 1  -1 -1 1   -1 1  0  4  1   -1 1  -1                 R3C4                                                                              -1 1   -1 1  0   -4 -1 1  -1 1   -1 1  2  2  -1  1  -1 1                  __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________    Data                                                                          ON -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1                 OFF                                                                              1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1     1                                                                      -1                      Strobe                                                                        R1  -3 -1  0  0  0   0  0  0  3  1   0  0  0  0  0   0  0  0                  R2  0  0   -3 -1 0   0  0  0  0  0   3  1  0  0  0   0  0  0                  R3  0  0   0  0  -3  -1 0  0  0  0   0  0  3  1  0   0  0  0                  R4  0  0   0  0  0   0  -3 -1 0  0   0  0  0  0  3   1  0  0                  Waveforms at x,y intersections for the display of FIG. 3                      R1C1                                                                              -2 -2  1  -1 1   -1 1  -1 4  0   1  -1 1  -1 1   -1 -2 -2                 R2C2                                                                              -1 1   -4 0  -1  1  -1 1  -1 1   2  2  -1 1  -1  1  -1 1                  R3C3                                                                              -1 1   -1 1  -2  -2 -1 1  -1 1   -1 1  4  0  -1  1  -1 1                  R3C4                                                                              1  -1  1  -1 -4  0  1  -1 1  -1  1  -1 2  2  1   -1 1  -1                 __________________________________________________________________________

                                      TABLE 5                                     __________________________________________________________________________    Data                                                                          ON  1  -1 1 -1                                                                              1  -1 1  -1 1  -1 1 -1                                                                              1  -1 1  -1 1  -1 1  -1 1                 OFF -1 1  -1  1                                                                             -1 1  -1 1  -1 1  -1  1                                                                             -1 1  -1 1  -1 1  -1 1   -1               Strobe                                                                        R1  1  -3 -1  3                                                                             0  0  0  0  0  0  0  0                                                                              0  0  0  0  1  -3 -1 3  0                 R2  0  0  0  0                                                                              1  -3 -1 3  0  0  0  0                                                                              0  0  0  0  0  0  0  0   1                R3  0  0  0  0                                                                              0  0  0  0  1  -3 -1  3                                                                             0  0  0  0  0  0  0  0   0                R4  0  0  0   0                                                                             0  0  0  0  0  0  0  0                                                                              1  -3 -1 3  0  0  0  0   0                Waveforms at x,y intersections for the display of FIG. 3                      R1C1                                                                              0  -2 -2  4                                                                             -1 1  -1 1  -1 1  -1  1                                                                             -1 1  -1 1  0  -2 -2 4  -1                R2C2                                                                              1  -1 1 -1                                                                              2  -4 0  2  1  -1 1 -1                                                                              1  -1 1  -1 1  -1 1  -1  2                R3C3                                                                              1  -1 1 -1                                                                              1  -1 1  -1 0  -2 -2  4                                                                             1  -1 1  -1 1  -1 1  -1  1                R3C4                                                                              -1 1  -1  1                                                                             -1 1  -1 1  2  -4 0  2                                                                              -1 1  -1 1  -1 1  -1 1   -1               __________________________________________________________________________

                                      TABLE 6                                     __________________________________________________________________________    Data                                                                          On  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1                      Off 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1                     Strobe                                                                        R1  -3 1  3  -1 0  0  0  0  0  0  0  0  0  0  0  0  -3 1                      R2  0  0  0  0  -3 1  3  -1 0  0  0  0  0  0  0  0  0  0                      R3  0  0  0  0  0  0  0  0  -3 1  3  -1 0  0  0  0  0  0                      R4  0  0  0  0  0  0  0  0  0  0  0  0  -3 1  3  -1 0  0                      Waveforms at x,y intersections for the display of FIG. 3                      R1C1                                                                              -2 0  4  -2 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 -2 0                      R2C2                                                                              -1 1  -1 1  -4 2  2  0  -1 1  -1 1  -1 1  -1 1  -1 1                      R3C3                                                                              -1 1  -1 1  -1 1  -1 1  -2 0  4  -2 -1 1  -1 1  -1 1                      R3C4                                                                              1  -1 1  -1 1  -1 1  -1 -4 2  2  0  1  -1 1  -1 1  -1                     __________________________________________________________________________

                                      TABLE 7                                     __________________________________________________________________________    Data                                                                          ON  1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1                     OFF -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1                      Strobe                                                                        R1  -1 -3 1  3  0  0  0  0  0  0  0  0  0  0  0  0  -1 -3                     R2  0  0  0  0  -1 -3 1  3  0  0  0  0  0  0  0  0  0  0                      R3  0  0  0  0  0  0  0  0  -1 -3 1  3  0  0  0  0  0  0                      R4  0  0  0  0  0  0  0  0  0  0  0  0  -1 -3 1  3  0  0                      Waveforms at x,y intersections for the display at FIG. 3                      R1C1                                                                              -2 -2 0  4  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -2 -2                     R2C2                                                                              1  -1 1  -1 0  -4 2  2  1  -1 1  -1 1  -1 1  -1 1  -1                     R3C3                                                                              1  -1 1  -1 1  -1 1  -1 -2 -2 0  4  1  -1 1  -1 1  -1                     R3C4                                                                              -1 1  -1 1  -1 1  -1 1  0  -4 2  2  -1 1  -1 1  -1 1                      __________________________________________________________________________

                                      TABLE 8                                     __________________________________________________________________________    Data                                                                          ON  1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1                     OFF -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 1                      Strobe                                                                        R1  -3 -1 3  1  0  0  0  0  0  0  0  0  0  0  0  0  -3 -1                     R2  0  0  0  0  -3 -1 3  1  0  0  0  0  0  0  0  0  0  0                      R3  0  0  0  0  0  0  0  0  -3 -1 3  1  0  0  0  0  0  0                      R4  0  0  0  0  0  0  0  0  0  0  0  0  -3 -1 3  1  0  0                      Waveforms at x,y intersections for the display of FIG. 3                      R1C1                                                                              -2 -2 4  0  1  -1 1  -1 1  -1 1  -1 1  -1 1  -1 -2 -2                     R2C2                                                                              -1 1  -1 1  -4 0  2  2  -1 1  -1 1  -1 1  -1 1  -1 1                      R3C3                                                                              -1 1  -1 1  -1 1  -1 1  -2 -2 4  0  -1 1  -1 1  -1 1                      R3C4                                                                              1  -1 1  -1 1  -1 1  -1 -4 0  2  2  1  -1 1  -1 1  -1                     __________________________________________________________________________

The curve shown in FIG. 5 is affected by a number of factors. For goodmultiplexing a curve with a minimum value of the V.t product isrequired. The minimum theoretical value of V.t for the materialsdescribed above is given as ##EQU1## where Ps is spontaneouspolarisation coeficient, εo=permittivity of free space

Δε=dielectric anisotropy of liquid crystal material

θ=cone angle of ferro electric liquid crystal material.

This applies to the case of homogeneous alignment of the liquid crystalmolecules. In a practical device where there is likely to be tilt in thebulk of the liquid crystal layer Emin is higher than this value.

FIG. 6 shows how the value of Emin is moved upwards and to the left asthe amount of applied A.C. voltage, i.e. the data voltage, is increased.The reason for this is the interaction of the applied field with thenegative dielectric anisotropy of the liquid crystal material. Suchinteraction tends to move the liquid crystal material from a tilted to amore homogeneous structure. The liquid crystal material used is LPM 68in a layer 1.7 μm thick at a temperature of 20° C.

FIG. 7 shows the effect of varying the amplitude and magnitude of theleading pulse in each pair of strobe pulses. The voltage at eachelectrode intersection, or pixel, is the difference between data andstrobe voltages is the resultant waveform. FIG. 8(a), (b) show theresultant waveform at a pixel when addressed by a strobe pulse pair anddata waveforms. In FIG. 8(a) the resultant waveform is a positive firstor leading pulse followed by a negative second or trailing pulse; thisis defined as a negative leading pulse ratio because the magnitudes areof opposite sign. A negative leading pulse followed by a positivetrailing pulse also has a negative leading pulse ratio. In contrast FIG.8(b) shows a waveform with both pulses of the same sign; this is definedas a positive leading pulse ratio. A zero leading pulse ratio will havea zero voltage level leading pulse. FIG. 7 shows V.t curves forresultant waveforms with leading pulse ratios of -0.5, -0.2, 0, 0.2, and0.5. The material and cell are as in FIG. 6 but at a temperature of 30°C. and with no A.C. bias. Region marked A is non switching (or partialswitching), region B is switching by the trailing pulse, and region C isswitching by leading pulse.

FIG. 9 shows how the V.t curve is affected by temperature. The curvesare for temperatures of 10°, 20°, 30°, and 40° C.; the cell material andthickness are as for FIG. 7. The value of Emin occurs at lower responsetimes but higher voltages as temperature increases.

Using the above changes in the V.t curve characteristics, temperaturecompensation can be built into the display of FIG. 1. This is achievedby measuring the temperature of the liquid crystal material with thethermocouple 15 (FIG. 1) and varying the amplitude and sign of theleading pulse in the strobe pulse pair. Using a negative leading pulseratio the value of Emin can be moved to a lower voltage at acorrespondingly higher response time. Using a positive leading pulseratio Emin can be moved to a faster response time at a correspondinglyhigher voltage.

By way of example a 16 by 16 pixel matrix cell was made using thematerial LPM 68 in a 1.7 μm thick layer constructed as for FIG. 2. Theapplied waveforms were as in FIG. 4 with data voltage Vd of 5 voltsamplitude, trailing strobe pulse voltage Tp of 40 volts, a variableleading pulse voltage Lp, and time slots of 60 μs whilst simulating 32way multiplexing. Temperature and leading pulse Lp were varied as inTable 9. A clear, good contrast, display was obtained at all temperaturepoints with the listed leading pulse voltages.

                  TABLE 9                                                         ______________________________________                                                                    Resultant                                         Temperature                 Waveform Ratio                                    °C.                                                                              Lp volts Lp/Tp Ratio  Vx    Vy                                      ______________________________________                                        15          4      0.1          -0.02 0.26                                    19.7       -4      -0.1         -0.2  +0.03                                   25.5       -8      -0.2                                                       30        -12      -0.3         -0.38 -0.2                                    34.1      -16      -0.4                                                       36.2      -20      -0.5                                                       38.3      -28      -0.7         -0.73 -0.66                                   39.4      -32      -0.8                                                       45        -40      -1.0         -0.78 -1.0                                    ______________________________________                                    

Vx, Vy=ratio of leading pulse to trailing pulse of resultant waveform inthe two strobe pulse pairs.

Taking the three temperature values of 19.7°, 30°, 38.3° C. the data,strobe, and resultant waveform are shown in the following table, usingthe format of Table 1 for a 4×4 matrix.

                                      TABLE 10                                    __________________________________________________________________________    Numbers are d.c. voltage levels                                               __________________________________________________________________________    Data 5  -5 5  -5 5  -5 5  -5 5  -5 5  -5                                      Temperature 19.7° C.                                                   Strobe                                                                             -4 40 0  0  0  0  0  0  4  -40                                                                              0  0                                       Resultant                                                                          -9 45 -5 5  -5 5  -5 5  -1 -35                                                                              -5 5                                       Temperature 30° C.                                                     Strobe                                                                             -12                                                                              40 0  0  0  0  0  0  12 -40                                                                              0  0                                       Resultant                                                                          -17                                                                              45 -5 5  -5 5  -5 5  7  -35                                                                              -5 5                                       Temperature 38.3° C.                                                   Strobe                                                                             -28                                                                              40 0  0  0  0  0  0  28 -40                                                                              0  0                                       Resultant                                                                          -33                                                                              45 -5 5  -5 5  -5 5  23 -35                                                                              - 5                                                                              5                                       __________________________________________________________________________

From this the result of a strobe pair pulse at 19.7° C. gives aresultant pulse pair of -9, 45 and later -1, -35. This gives a leadingpulse ratio of -9/45=-0.2, and -1/-35=0.03. Note these two ratios arethe same when the inverse of the data waveform is used. The datawaveform and its inverse are used depending upon whether a pixel is tobe switched to an ON or OFF state. The leading pulse ratios can becalculated for the other temperature values; the results are given inTable 9.

Taking the leading pulse ratios in Table 9 V.t plots have beendetermined for the three temperatures 19.7°, 30°, 38.3° C. and theresults are shown in FIGS. 10, 11, 12 respectively. Each case curve Ashows the response to the first strobe pulse pair, and curve B theresponse to the second strobe pulse pair.

Looking first at FIG. 10 the first strobe pulse pair gives a resultantwaveform of -9 then 45 volts, i.e. a leading pulse ratio of -0.2, andcurve A applies. Thus a voltage of 45 (preceded by -9) for less thanabout 700 μs will not switch. Looking now at the second strobe pulsepair the resultant waveform is -1 then -35 volts, i.e. a leading pulseratio of 0.03, and curve B applies. Thus a voltage of (-)35 preceded by(-)1 will switch the material if the slot time is greater than about 80μs. The voltage levels of 45 and (-)35 are be marked on FIG. 10 asvertical lines with a band of time slots. Clear and clean switching isobtained for time slots of about 70 to 400 μs. The bands start slightlybelow the V.t curves because in practice optical switching is observedat the marked values.

Similarly in FIG. 11 curve A applies to the resultant waveform of thefirst strobe pulse pair where Vx=-0.38, and curve B applies to thesecond strobe pulse pair where Vy=-0.2. A voltage of 45 volts, precededby -17 volts, does not switch providing the time slot is less than about180 μs. A voltage of -35 preceded by 7 volts switches providing the timeslot is greater than about 80 μs. Clear and clean switching is availablefor time slots of about 80 to 180 μs.

Two additional curves are marked C, D for the resultant leading pulseratios of -0.32 and -0.2 respectively. The C, D curves are plots of thetrailing pulse V.t values for resultant pulse pairs that switch the cellon leading pulses. This contrasts with the previous resultant waveformswhere the cell always switched on a trailing pulse. It seemsunpredictable that a cell should switch on receipt of a small resultantleading pulse and not switch on the larger value trailing pulse.However, this is an observed phenomenon and is due to molecules relaxingimmediately prior to receiving the leading pulse. After such relaxationthe small leading pulse is able to switch itself fully, but the cellcannot fully switch again within the available time slot of the largeramplitude trailing pulse.

For example a given pixel switched by a -35 volts, preceeded by 7 volts(curve B) also receives 45 volts preceeded by -35 volts and no switchingon the trailing pulse of 45 volts occurs because it is below curve A.However, 45 volts lies within the switching area of curve C for timeslots of about 130-180 μsecs. Thus the leading pulse of -35 voltspreceeding 45 volts switches or reinforces the given pixel also switchedto the same state by the -35 volts trailing pulse. The net effect ofcurves C, D in FIG. 11 is to reinforce the switching already describedfor curves A, B within a limited range of time slots.

Again in FIG. 12 curve A applies to the resultant waveform of the firststrobe pulse pair where Vx=-0.73, and curve B applies to the secondstrobe pulse pair where Vy=-0.66. A voltage of 45 volts, preceded by -33volts, does not switch providing the time slot is less than about 80 μs.A voltage of -35 preceded by 23 volts switches providing the time slotis greater than about 63 μs. Clear and clean switching is available fortime slots of about 63 to 80 μs. Curves C, D show curves for leadingpulse switching as in FIG. 11. These reinforce the leading pulseswitching of curves A, B.

Not shown by Figures but listed in Table 9 are details obtained for thetemperature 15° C. This was found to be multiplex addressable for timeslot periods of about 70 to 200 μs.

The above shows how a given cell can be satisfactorily addressed over atemperature range of 10° to 40° C. merely by changing the amplitude ofthe leading strobe pulse in each strobe pair from +8 volts to -32 volts,the + or - sign representing the same or opposite polarity as thetrailing pulse voltage of +40 volts. These values represent leadingpulse ratios Lp/Tp of +0.2 to -0.8.

As a further example the above cell with material LPM 68 was operatedunder the following conditions and the following results obtained:

Strobe trailing pulse voltage Vs=15 volts, data pulse Vd=5 volts, and a120 μs time slot.

                  TABLE 11                                                        ______________________________________                                                  Leading                                                             Temperature                                                                             pulse volts                                                                             Lp/Tp ratio  Vx    Vy                                     ______________________________________                                        15        12        0.8          0.35  1.7                                    20        5         0.33         0     1.0                                    25        0         -0.25        -0.25 0.5                                    30        -6        -0.4         -0.55 -0.1                                   35        -15       -1           -1    -1                                     ______________________________________                                    

Note the levels of resultant voltages are below Emin on the graphs ofFIGS. 6 to 11. Temperature compensation is applicable for displaysoperating both above and below Emin.

Thus to provide compensation for liquid crystal temperature variationthe strobe waveform generator is programmed to output strobe pulses witha ratio that varies with the liquid crystal temperature. Differentmaterials and cell thickness will have different characteristics thatneed to be predetermined.

Observation of Tables 9 and 11 show the Lp/Tp ratio to be approximatelylinearly related to Temperature. Thus the output of the thermocouple 15can be fed to an inverting amplifier for controlling the amplitude ofthe leading pulse in each strobe pair. Alternatively a ROM chip can beprogrammed to output the required leading pulse voltage level for apredetermined set of different temperatures inputs.

All the above strobe waveforms use identical but opposite polarity firstand second pulse pairs. In a modification the strobe leading pulse ratioLp/Tp is varied between the first and second pulse pair. This has theeffect of increasing the separation between the curves A, B in FIGS. 10to 12. The resulting small d.c. bias is removed by periodicallyreversing display polarity.

In a modification the values of the data pulse pair may be varied infield 1 and field 2 to improve the separation of curves A and B in FIGS.10-12. This may be achieved either in conjunction with variation of theleading part of the strobe pulse pair or independently of it and maytake a number of forms:

(i) an equal reduction in amplitude of each of the first pair of datapulses with a corresponding increase in the amplitude of the secondpair;

(ii) an equal increase in amplitude of each of the first pair of datapulses with a corresponding decrease in the amplitude of the secondpair;

(iii) an increase in the amplitude of the first pulse of the first pairof data pulses with a corresponding decrease in amplitude of the firstpulse of the second pair;

(iv) a decrease in the amplitude of the first pulse of the first pair ofdata pulses with a corresponding increase in amplitude of the firstpulse of the second pair

(v) and (vi) vary second pulse of the pair.

In a further modification the first strobe pair is replaced by ablanking pulse that completely switches to one state a line at a time.Alternatively a group of lines or the whole display can be blanked atone time. Pixels requiring to be switched to the other state areswitched by the remaining strobe pulse pair. The resulting d.c. bias isremoved by periodically reversing polarity. Use of blanking eliminatesthe first field in the addressing and reduces the complete addressingtime.

I claim:
 1. A multiple addressed liquid crystal display comprising:aliquid crystal cell including a layer of ferro-electric smectic liquidcrystal material contained between two walls, each wall bearing a set ofelectrodes, said electrodes in combination comprising a matrix ofaddressable intersections, driver circuits for applying data waveformsto one set of electrodes and strobe waveforms to the other set ofelectrodes in a multiplexed manner, waveform generators for generatingdata and strobe waveforms for applying to the drive circuits, means forcontrolling the order of data waveforms so that a desired displaypattern is obtained, said waveform generators including: a data waveformgenerator means for generating two continous sets of data waveforms ofequal amplitude and frequency but opposite sign, each data waveformcomprising continous d.c. pulses of alternate sign, each pulse having asingle time slot duration ts; and a strobe waveform generator means forgenerating strobe waveforms comprising a first pair of strobe pulses ofdifferent amplitude followed by a second pair of pulses of similaramplitude but different sign to the first pair of strobe pulses, eachstrobe pulse having a duration coincident with and equal to said timeslot duration ts.
 2. The display of claim 1 wherein the strobe waveformgenerated by said strobe waveform generator means comprises two pairs ofstrobe pulses separated from one another by a number of time periodswhen a zero strobe pulse is generated.
 3. The display of claim 1 whereinthe strobe waveform generated by said strobe waveform generator meanscomprises two pairs of strobe pulses immediately following one anotherin time.
 4. The display of claim 1 wherein said strobe waveformgenerator means includes means for varying at least one of amplitude andsign of a leading pulse with reference to a trailing pulse in eachstrobe pulse pair.
 5. The display of claim 1 further comprising atemperature sensing element for sensing the liquid crystal layertemperature, and means for varying amplitude and sign of the leadingpulse voltage in each strobe pulse pair to compensate for temperaturevariation in the liquid crystal layer.
 6. The display of claim 1 whereinsaid strobe waveform generator means includes means for independentlyvarying at least one of amplitude and sign of a leading pulse in eachstrobe pulse for compensation of temperature variation in the liquidcrystal material.
 7. The display of claim 1 wherein said data waveformgenerator means includes means for varying amplitude of the datawaveform.
 8. A method of multiplex addressing a ferro electric liquidcrystal matrix display formed by the intersections of a first set ofelectrodes and a second set of electrodes, said method comprising thesteps of:applying a strobe waveform to each electrode in sequence in thefirst set of electrodes, said strobe waveform comprising a first pair ofstrobe pulses of different amplitude followed by a second pair of pulsesof similar amplitude but different sign to the first pair of strobepulses, each strobe pulse lasting a single time slot duration ts;applying one of two data waveforms to each electrode in the second setof electrodes coincidentally with strobe waveform, both data waveformsbeing rectangular waveforms of alternate positive and negative valueswith one data waveform the inverse of the other data waveform, each datawaveform lasting a single time slot duration ts; whereby eachintersection is addressed with a d.c. pulse of appropriate sign andmagnitude to turn that intersection to a desired display sate once percomplete display address period and an overall net zero d.c. value ineach complete display address period.
 9. The method of claim 8 whereinthe leading pulse in each strobe pulse pair is varied in amplitude andsign to compensate for temperature variation in the liquid crystalmaterial.
 10. The method of claim 8 wherein the amplitude of the datawaveform is varied to compensate for temperature variation in the liquidcrystal material.
 11. The method of claim 8 wherein the values ofapplied voltage and time of application product (V.t) are arranged sothat the liquid crystal material switches to a given state on receipt ofthe trailing pulse in one pulse pair and also switches to the same stateon receipt of the leading pulse in a different pulse pair.
 12. Amultiple addressed liquid crystal display comprising:a liquid crystalcell including a layer of ferro-electric smectic liquid crystal materialcontained between two walls each bearing a set of electrodes, saidelectrodes in combination comprising a matrix of addressableintersections; driver circuits for applying data waveforms to one set ofelectrodes and strobe waveforms to the other set of electrodes in amultiplexed manner; waveform generators for generating data and strobewaveforms for applying to the driver circuits; means for controlling theorder of data waveforms so that a desired display pattern is obtained;and means for sensing the liquid crystal temperature, said waveformgenerators include a data waveform generator means for generating twosets of data waveforms of equal amplitude and frequency but oppositesign, each data waveform comprising d.c. pulses of alternate sign, eachpulse lasting for a single time slot duration ts and a strobe waveformgenerator means, responsive to said temperature sensing means, forgenerating strobe waveforms comprising a first pair of strobe pulses ofdifferent amplitude followed by a second pair of pulses of similaramplitude but different sign to the first pair of strobe pulses, eachstrobe pulse having a duration coincident with and equal to said timeslot duration ts, where amplitude and sign of a leading pulse in eachstrobe pulse pair is independently variable in response to sensed liquidcrystal temperature to compensate for changes in liquid crystaltemperature.